Field of Invention
The present disclosure relates generally to optical systems, more particularly it relates an optical system such as a macro lens, and to an image pickup apparatus including the optical system, such as a digital still camera, a video camera, a monitoring camera, or a broadcasting camera, or a camera using a silver-halide film.
Description of Related Art
Recently, there has been an increasing need for macro photography to capture an image using an image pickup apparatus, such as a digital camera or a video camera. Macro photography is a technique used to capture an image of a small object, such as snow crystals or an insect head while magnifying the object. To enable macro photography, an optical system having high imaging magnification is required.
One known technique for increasing the imaging magnification includes the use of a rear conversion lens. According to this known technique, a rear conversion lens is attached on an image side of the imaging lens. Japanese Patent Application Laid-Open No. 63-205627 discusses a rear conversion lens that is inserted between the main lens system and the image plane. Use of the rear conversion lens in addition to the main lens system can achieve an increase in imaging magnification as compared to use of only the main lens system.
In general, an increase in imaging magnification tends to cause degradation in optical performance of the optical system. This makes it difficult to increase the imaging magnification of an optical system, and at the same time, to maintain high optical performance. In Japanese Patent Application Laid-Open No. 63-205627, an imaging lens achieves an increased imaging magnification and high optical performance at the same time by increasing refractive power of a rear conversion lens and by increasing the number of lens elements included in the rear conversion lens.
However, in Japanese Patent Application Laid-Open No. 63-205627, the increased number of lens elements included in the rear conversion lens of the image-capturing optical system leads to an increase in the thickness of the rear conversion lens. This in turn is likely to increase the overall size of the image-capturing optical system. In addition, the increased refractive power of the rear conversion lens tends to cause aberration, such as curvature of field, which further contributes to degradation in optical performance and hinders imaging quality.